| 5 | |
| 6 | |
| 7 | class Stack(list): |
| 8 | def is_empty(self): |
| 9 | return len(self) == 0 |
| 10 | |
| 11 | @property |
| 12 | def top(self): |
| 13 | if self.is_empty(): |
| 14 | return None |
| 15 | return self[-1] |
| 16 | |
| 17 | @property |
| 18 | def bottom(self): |
| 19 | if self.is_empty(): |
| 20 | return None |
| 21 | return self[0] |
| 22 | |
| 23 | def size(self): |
| 24 | return len(self) |
| 25 | |
| 26 | def push(self, item): |
| 27 | self.append(item) |
| 28 | |
| 29 | |
| 30 | class QuerySetChain: |